1. Field of the Invention
The invention relates to a multilayer material comprising a bearing material applied directly to a backing material by sputtering, which bearing material comprises a matrix material of copper or a copper-based alloy with finely dispersed lead inclusions.
2. Description of Related Art
Copper-based materials with various alloy elements are widely used as highly-loadable plain bearing materials in modern combustion engines. Their considerable strength makes them suitable for use as connecting-rod bearings, main bearings, piston pin bushings and rocker bushings as well as for use as gear parts or as components in machine construction in general. As the lead content increases and the tin content decreases, for example, conformability to and compatibility with the counter-member increases, whereas corrosion resistance and strength decrease markedly. For use as connecting-rod or main bearings in modern engine construction, these bearings are therefore generally provided with a third electrodeposited layer, which markedly improves corrosion resistance. Connecting-rod bearings and crank bearings constructed in this way find millions of applications in the engine construction industry. However, the three-layer structure of these multilayer materials is technically extremely complex.
It is known from "Gleitlager", E. Schmidt, R. Weber, 1953, p. 192, that with lead-bronzes there is a straightforward correlation between structural formation or structural shape and corrosion behaviour. This means that the more finely crystalline the structure and thus also the more finely the lead particles are dispersed within the matrix, the more corrosion-resistant the material. In addition, it is advantageous for the lead particles to exhibit as fine a globular structure as possible.
To counter effectively the loss in matrix strength as the lead content increases, efforts have been made to achieve as finely crystalline a structure as possible, but lower limits are set to the achievement thereof by for example casting methods. The very much more favourable strength values achieved with finely crystalline metallic materials compared with those which are coarsely crystalline are based on an effect which markedly increases strength and which may be understood from the following background described in "Physikalische Metallkunde", Peter Haasen, 2nd Edition 1984, p. 246:
In metallic materials, deformation is caused by the migration of lattice defects (dislocations). If such a material is present which has as finely crystalline a structure as possible, i.e. a high grain boundary content, then, if deformation and the consequent migration of dislocations occur, the latter become pinned at the grain boundaries and act as obstacles. High internal stress fields are induced thereby, which prevent or at least make more difficult any further migration of the dislocations. This state of affairs results in a direct correlation between grain size and strength in metallic materials by way of the Hall-Petch equation, this so-called grain boundary consolidation increasing as the grain diameter d falls according to d.sup.-0.5.
These structural requirements in the production of sliding materials may be fulfilled outstandingly by the application of PVD technology (sputtering) to the deposition of this group of materials.
It is known from DE 28 53 724 that the cathodic evaporation of metals, especially AlSnCu alloys, makes it possible to produce overlays which exhibit much higher hardness levels than cast materials of the same chemical composition, and consequently outstanding wear resistance. This higher strength is provided by hard oxide particles finely dispersed in the layer, which result in dispersion strengthening ensuring, particularly at relatively high temperatures, that the mechanical characteristic values, such as high-temperature strength and high-temperature wear resistance, do not drop noticeably, as is known to occur with casting alloys. In the case of this known multilayer material, in which AlSn.sub.20 Cu.sub.1 is preferably sputtered onto a bronze layer, e.g. of CuPb.sub.22 Sn, a diffusion barrier layer is necessary. Diffusion barrier layers of chromium-nickel alloys are proposed therein, because they are also easy to sputter and are distinguished by outstanding adhesion to the backing and a high level of efficacy as a diffusion barrier layer. The provision of an additional diffusion barrier layer is expensive, however. Moreover, although these known multilayer materials are very hard, they exhibit deficits as far as conformability and embedding properties are concerned.
In order to be able to dispense with these nickel-chromium intermediate layers, it was proposed according to EP 0300 993 to construct the sputtered layer in such a way that the matrix material exhibits excellent columnar growth, in the interstitial spaces of which the soft phases are incorporated in fine dispersion. Owing to this columnar growth, which is adjusted by means of the process parameters, it should also be possible to improve abrasion resistance. However, it has proven to be a disadvantage that the columnar crystallites are relatively large, which makes it impossible to achieve a high grain boundary content and thereby results in inadequate strength levels. Furthermore, the embedding properties of the bearing materials is unsatisfactory.
DE 3629451 A1 discloses a composite material which includes in the matrix an insoluble material with a specific particle diameter, the average value of which is x&lt;0.8 .mu.m. Normal dispersion with the average value x is clearly obtained by deliberately bestowing special properties on the overlay which consist in the temperature of the substrate remaining below 150.degree. C. during coating. Moreover, an elevated coating speed also contributes thereto. These process features are all conformed to the particle size of the lead particles and not to the crystal structure of the matrix particles, about which no further details are given. The structure of the overlay obtained ensures maintenance of the hardness even after heat treatment lasting 300 hours.
DE 4142454 A1 does not describe any sputtered overlays, but rather recommends the use of plasma arc hardfacing for lead-containing materials. In this process, the temperature loading is greater than is the case with sputtering, and much thicker layers are produced which have then also to be machined. The overlays produced according to this process contain from 5 to 40 wt. % lead, the lead particles having a diameter of up to 50 .mu.m.
DE 3721008 A1 describes a multilayer material in which hard particles are deliberately incorporated, and DE 3813801 A1 relates to a multilayer material with a functional layer applied to a backing layer, which functional layer comprises a thin, uninterruptedly closed surface region in which the dispersion is provided, by melting and rapid cooling from the molten state, with a structure which is improved with respect to the rest of the functional layer and exhibits fine-particled distribution of the undissolved components. The functional layer has in itself a coarse structure, such that structural alterations need be effected by additional measures only in a superficial area.